1. Field of Invention
The present invention relates to an optical circulator and, in particular, to an optical circulator that couples optical fibers with optical devices and can be applied to optical fiber transmission of optical signals.
2. Related Art
An optical circulator is a passive device that has at least three ports for accepting optical fibers. It is featured in that light that enters the circulator through the first port exits through the second port, and light that enters through the second port exits through the third. When the number of ports increases, this principle stays the same. That is, the optical path is not retraceable in the optical circulator, light that enters the nth port exits through the (n+1)th port.
Circulators are used for fiber transmission of optical signals. For example, the first ports of two optical circulators may be connected to a data transmitter, the second ports to an optical fiber, and the third ports to a data receiver. The same fiber is then used for transmitting and receiving signals.
For anisotropic birefringent crystals, incident light can be classified into extraordinary ray (E-ray) and ordinary ray (O-ray) according to its polarization direction and those two polarization directions are orthogonal. For a linearly polarized beam, the two polarization directions differ by 90 degrees. The O-ray will obey the Snell""s law and the wave propagating direction will be parallel to the energy propagating direction. However, the wave propagating direction of the E-ray is normally not parallel to the O-ray and the energy propagating direction usually differs due to the crystal optical axis. This is called the walk-off phenomenon.
When light passes through a reciprocal crystal in the forward optical path, the polarization direction will be rotated by a certain angle; whereas when the light passes through the reciprocal crystal again in the returning path, the polarization direction will be rotated back by the same angle. So the polarization of the light is not changed after the round trip. On the other hand, when light passes through a non-reciprocal crystal in the forward optical path, the polarization direction is rotated by a certain angle; whereas when the light passes through the non-reciprocal crystal in the returning path again, the polarization direction is rotated further by the same angle. Therefore, the change in the polarization of the light is additive in the round trip of the beam. A proper combination of reciprocal crystals and non-reciprocal crystals can generate a particular linearly polarized direction and allow the choice of producing the walk-off phenomenon in order to achieve the above goal of an irretraceable optical path inside the optical circulator. Normal optical circulators use half-wave plates as the reciprocal crystals but not the Faraday rotators, which are non-reciprocal crystals.
The design of optical circulators is in whether each port can be distinguished from one another by its axial direction. When different ports of the optical circulator are not in the same axial direction, a polarizing beam splitter (PBS) has to be employed. The product occupies a large volume and costs more, e.g. the technology disclosed in the U.S Pat. No. 5,878,176. To reduce the volume and cost of the product, having different ports in the same axial direction has become the trend of modern designs; see for example the U.S. Pat. No. 5,930,422. Based upon the consideration of lower costs and convenient assembly, the U.S. Pat. No. 5,973,832 discloses a technology to remove half-wave plates by using the relative angle between a multi-layer Faraday rotator and a birefringent crystal optical axis. The U.S. Pat. No. 6,002,512 discloses a technology to reduce the number of half-wave plates used by employing latchable Faraday rotators. The U.S. Pat. No. 5,930,039 discloses a two-core fiber collimator that makes the three ports only need two optical fiber collimators, greatly minimizing the product volume and lowering the manufacturing cost.
The object of the invention is to provide an optical circulator that reduces the volume, lower the cost and solve such problems as the conjugate angle of the two-core collimator and the minimal polarization mode dispersion.
According to the technology disclosed herein, the first port and the second port of the optical circulator are both become a two-core optical fiber collimator using a reflector. Since the same crystal is used repeatedly in the optical path, the volume of the optical circulator can be decreased and the assembly procedure can be simplified. It can further conquer such problems as the conjugate angle of the two-core collimator and the minimal polarization mode dispersion (PMD). Using the property that the polarization state of a light beam will not change when passing through a reciprocal crystal back and forth once, while will change additively when passing through a non-reciprocal crystal back and forth once, the present invention properly combine reciprocal crystals and non-reciprocal crystals to generate a particular polarization direction, to allow the choice of producing the walk-off phenomenon, and to form an optical circulator with an irretraceable optical path therein. In particular, the corresponding relation between the Faraday rotator and the birefringent crystal optical axis can be utilized to remove half-wave plates used in ordinary optical circulators, thus lowering manufacturing costs and complexities.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.